Prevalence Rates & Disorders (SL IB Psychology)

• Prevalence rates refer to how common a specific disorder is within a defined population

• Prevalence for, say, major depressive disorder (MDD) would be measured as the proportion of people in the UK who were diagnosed with MDD from, say, January 2021 to January 2022
• Prevalence rates are measured using an exact, mathematical calculation as follows:
  • The total number of cases of MDD in a specific population is identified
  • This number is then divided by the total number of people from that specific population
  • An example of a (hypothetical) prevalence rate might be: in a population of 50,000 people 4,500 have been diagnosed with MDD which means that the prevalence rate for MDD within that population is 0.09 (4,500 ÷ 50,000 = 0.09) or 9,000 cases of MDD per 100,000 people
• Prevalence is a measure of an illness that enables statisticians to determine the likelihood of any person within that population being diagnosed with that illness e.g. it is a predictive tool

• MDD has a high prevalence across the world: the World Health Organisation (WHO) has estimated that 3.8% of the world population suffers from MDD at any given time (5% for adults and 5.7% for those in the 60+ age group)
• The above figures translate to a total of 280,000,000 people with MDD out of the total world population
• The prevalence rate for MDD in the UK is 0.17 (this accounts for 1 in 6 adults with MDD across the population of the UK)
• MDD is more prevalent in women both in the UK and globally (15% of women seek treatment for MDD compared to 9% of men)
• There are cultural variations in MDD prevalence: 1% in the Czech Republic; 9% in Chile; 16.9% in the USA

• Phobias have a worldwide prevalence rate of 7.4% according to the WHO with more women than men suffering from phobias
• According to the National Institute of Mental Health (USA) an estimated 9.1% of adults in the US suffer from having a specific phobia at any given time
• The rate of phobia prevalence is higher for women (12.2%) than for men (5.8%)
• The data also found that an estimated 12.5% of American adults will experience a specific phobia at some point in their lives
• The most prevalent phobia in the UK is fear of heights (acrophobia) as of September 2023 according to government statistics with 75% of those surveyed expressing some degree of fear related to high places
• Other highly prevalent phobias in the UK are fear of snakes (ophidiophobia) at 52%; fear of flying (aerophobia) at 24% and fear of spiders (arachnophobia) at 18%

Some research suggests that prevalence rates for anxiety disorders such as phobias are higher than for other anxiety disorders such as PTSD and OCD.

Which studies investigate prevalence rates of MDD and phobias?

• Abdoli et al. (2022) - prevalence rates of MDD

• Frederikson et al. (1996) - prevalence rates of phobias

Both Abdoli et al. and Frederikson et al. (1996) can be found as Two Key Studies of Prevalence Rates - just navigate the Etiology of Disorders section of this site.